Generally, the relationship between the apical curvature of an ophthalmic lens at which the astigmatic aberration of the light rays passing through the lens shows a minimum value and the apical refractive power on the optical axis of the lens (hereafter referred to as the “dioptric power”) can be obtained, for example, from the Tscherning ellipse.
Specifically, it is well known that the occurrence of astigmatic aberration in the peripheral parts of the lens can be suppressed by adjusting the curvatures of both refractive surfaces of the lens to an optimal combination obtained by means of this Tscherning ellipse, and the optical performance drops conspicuously if a combination of curvatures of both refractive surfaces that departs greatly from this Tscherning ellipse is selected.
However, in cases where an optimal combination of curvatures of both refractive surfaces obtained by means of this Tscherning ellipse is used, the curvatures of the refractive surfaces of the lens surfaces are increased, and the thickness of the lens also tends to increase.
Accordingly, in ophthalmic lenses seen in recent years, because of problems in terms of reducing the lens thickness and problems in terms of external appearance, and also for convenience of manufacture and the like, curvatures that are smaller than the curvatures obtained by the optimal combination of curvatures have been selected in almost all cases as the refractive surface curvatures (hereafter called the “base curve”) of lens surfaces on the object side (hereafter called the “outer surfaces”).
Specifically, in cases where problems of optical performance and external appearance are taken into account, the practical range of the base curve of an ophthalmic lens is limited to a specified range in accordance with the dioptric power of the ophthalmic lens. Furthermore, in cases where the refractive surfaces are constructed only from spherical surfaces, it is impossible to solve the problems of maintaining the optical performance in the desired range and improving the external appearance at the same time. Currently, therefore, lenses in which astigmatic aberration and dioptric power error in the peripheral parts of the lens are corrected while reducing the curvature of the base curve and maintaining the thickness of the lens at a small value mainly by making one of the refractive surfaces of the lens aspherical constitute the mainstream of lenses.
Ordinarily, in the principal rays passing through the ophthalmic lens corresponding to the line of sight of the user, the astigmatic aberration of the principal rays can be expressed as follows where Dmax is the maximum principal refractive power and Dmin is the minimum principal refractive power.|Dmax−Dmin|Furthermore, the mean refractive power of the principal rays can be expressed as follows:(Dmax+Dmin)/2These are treated as important factors in the design of the ophthalmic lens. Accordingly, in conventional ophthalmic lenses, the correction of aberration has been accomplished using such astigmatic aberration, mean refractive power, maximum principal refractive power and minimum principal refractive power as parameters.
Incidentally it is a well-known fact that the refractive power in the direction of an arbitrary meridian on a plane perpendicular to the principal rays has maximum and minimum values, and that the meridian directions that give these maximum and minimum values are perpendicular to each other. Accordingly, in the present specification, the maximum value of the refractive power in these arbitrary principal rays is taken as the maximum principal refractive power, the minimum value of this refractive power is taken as the minimum principal refractive power, and this maximum principal refractive power and minimum principal refractive power are referred to collectively as the principal refractive power.
Furthermore, the direction of the principal meridian including the maximum principal refractive power is taken as the maximum principal direction, the direction of the principal meridian including the minimum principal refractive power is taken as the minimum principal direction, and the maximum principal direction and minimum principal direction are referred to collectively as the principal direction. Moreover, in the present invention, the units of the values that express the refractive power are all diopter units unless otherwise specifically noted.
In a conventional ophthalmic lens, because of the need for cost reduction and simplification from the standpoint of manufacture, a semi-product lens (hereafter referred to as a semi-finished lens) in which one of the refractive surfaces of the lens has been worked beforehand is used. The refractive surface that has not been worked beforehand is called the prescription surface. Specifically, by working the prescription surface of the semi-finished lens into a spherical or toric surface in accordance with the prescription of the ophthalmic lens user, it is possible to use the same semi-finished lens in common in a fixed diopter range. Accordingly, this plays a large role in cost reduction (reduction of working costs, warehousing, and the like).
Ordinarily, the shape of the refractive surface of the semi-finished lens whose working is finished beforehand is a spherical surface shape or an aspherical surface shape that is rotationally symmetrical with respect to the optical axis, and this spherical surface shape or aspherical surface shape is set as a spherical or aspherical surface shape with respect to a certain specified dioptric power within the common dioptric power range so that the aberration shows a minimum value. If this set dioptric power is taken as the standard dioptric power for this semi-finished lens, then the optical performance of the lens in the dioptric power range in the vicinity of this standard dioptric power is favorable; however, in a dioptric power range that is removed from this standard dioptric power, a deterioration in the optical performance especially in the peripheral parts of the lens cannot be avoided. Furthermore, the following drawback is also encountered: namely, in cases where correction of astigmatism is necessary, even if the dioptric power is the standard dioptric power, a deterioration in the optical performance in the peripheral parts of the lens cannot be avoided.
Recently, however, because of the development of techniques for freely working aspherical surfaces, it has also become possible to correct aberration by forming prescription surfaces that have conventionally been spherical or toric surfaces into aspherical surfaces, so that ophthalmic lenses in which the conditions of use by the user are taken into account and the prescription surfaces are made aspherical have been developed into products.
However, especially in the case of ophthalmic lenses that involve correction of astigmatism, the astigmatic dioptric power according to the prescription is included in the astigmatic aberration of the lens, and the principal direction of the astigmatic axis of the eye, which varies with the movement of the line of sight, must be taken into account.
In order to provide an ophthalmic lens that is optimal for the individual user in accordance with specifications and conditions of use that vary with each user, the mere evaluation of the optical performance according to the meridional image plane and sagittal image plane, astigmatic aberration and mean refractive power conventionally used in the design of ophthalmic lenses is insufficient. In addition to the conventional evaluation of optical performance, torsion along the astigmatic axis of the eye and the principal direction of the principal refractive power of the lens must be taken into account, and a plurality of different types of optical performance must be simultaneously improved, in order to provide an ophthalmic lens that is optimal for each individual user. However, a so-called trade-off relationship may exist among different types of optical performance; consequently, it is difficult to reduce all aberration to a minimum value. Accordingly, it is necessary to achieve a refractive surface design which is such that a balance is obtained between different types of optical performance, so that an optimal optical performance is obtained overall.
Conventionally, in order to obtain a balance between different types of optical performance, designers have considered and judged various performance values. However, in order to design ophthalmic lenses by optimizing the optical performance for individual users, it is also necessary to automate such judgments by means of a computer.
The present invention was devised in light of such circumstances; it is an object of the present invention to provide a method for designing prescription surfaces so that a favorable optical performance can be obtained in various specifications accompanying individual information such as prescriptions for ophthalmic lens users, and an ophthalmic lens manufacturing method using this design method.